Multi threshold voltage for nanosheet

ABSTRACT

A semiconductor structure including nanosheet stacks on a substrate, each nanosheet stack including alternating layers of sacrificial semiconductor material and semiconductor channel material and a crystallized gate dielectric layer surrounding the semiconductor channel layers of a first subset of the nanosheet stacks, a dipole layer on top of the crystallized gate dielectric and surrounding the layers of semiconductor channel material of the first subset of the nanosheet stacks and a gate dielectric modified by a diffused dipole material surrounding the semiconductor channel layers of a second subset of the nanosheet stacks. A method including forming nanosheet stacks on a substrate, each nanosheet stack including alternating layers of sacrificial semiconductor material and semiconductor channel material, removing sacrificial semiconductor material layers of the set of nanosheet stacks, forming a gate dielectric surrounding the semiconductor channel layers of the nanosheet stacks, and crystalizing the gate dielectric of a subset of the nanosheet stacks.

BACKGROUND

The present invention relates, generally, to the field of semiconductormanufacturing, and more particularly to fabricating field effecttransistors.

Complementary Metal-oxide-semiconductor (CMOS) technology is commonlyused for field effect transistors (hereinafter “FET”) as part ofadvanced integrated circuits (hereinafter “IC”), such as centralprocessing units (hereinafter “CPUs”), memory, storage devices, and thelike. As demands to reduce the dimensions of transistor devicescontinue, nanosheet FETs help achieve a reduced FET device footprintwhile maintaining FET device performance. A nanosheet FET includes aplurality of stacked nanosheets extending between a pair of source drainepitaxial regions. The device may be a gate all around device ortransistor in which the gate surrounds a portion of the nanosheetchannel. A nanosheet device contains one or more layers of semiconductorchannel material portions having a vertical thickness that issubstantially less than its width.

Limited space between channels of a nanosheet device makes it difficultto adjust the threshold voltage using conventional techniques. It wouldbe advantageous to have more than one threshold voltage in asemiconductor structure for increased design flexibility ofsemiconductor devices in the semiconductor structure.

SUMMARY

According to an embodiment, a semiconductor structure is provided. Thesemiconductor structure including nanosheet stacks on a substrate, eachnanosheet stack including alternating layers of a sacrificialsemiconductor material and a semiconductor channel material verticallyaligned and stacked one on top of another, and a crystallized gatedielectric layer surrounding the semiconductor channel layers of a firstsubset of the nanosheet stacks, a dipole layer on top of thecrystallized gate dielectric and surrounding the layers of semiconductorchannel material of the first subset of the nanosheet stacks and a gatedielectric modified by a diffused dipole material surrounding thesemiconductor channel layers of a second subset of the nanosheet stacks.

According to an embodiment, a semiconductor structure is provided. Thesemiconductor structure including nanosheet stacks on a substrate, eachnanosheet stack including alternating layers of a sacrificialsemiconductor material and a semiconductor channel material verticallyaligned and stacked one on top of another, and a crystallized gatedielectric surrounding the semiconductor channel layers of the nanosheetstacks.

According to an embodiment, a method is provided. The method includingforming nanosheet stacks on a substrate, each nanosheet stack includingalternating layers of a sacrificial semiconductor material and asemiconductor channel material vertically aligned and stacked one on topof another, removing the sacrificial semiconductor material layers ofthe set of nanosheet stacks, forming a gate dielectric surrounding thesemiconductor channel layers of the nanosheet stacks, and crystalizingthe gate dielectric of a subset of the nanosheet stacks.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will become apparent from the following detailed descriptionof illustrative embodiments thereof, which is to be read in connectionwith the accompanying drawings. The various features of the drawings arenot to scale as the illustrations are for clarity in facilitating oneskilled in the art in understanding the invention in conjunction withthe detailed description. In the drawings:

FIG. 1 illustrates a cross-sectional view of a semiconductor structureat an intermediate stage of fabrication, according to an exemplaryembodiment;

FIG. 2 illustrates a cross-sectional view of the semiconductor structureand illustrates selective removal of sacrificial semiconductor materiallayers, according to an exemplary embodiment;

FIG. 3 illustrates a cross-sectional view of the semiconductor structureand illustrates formation of a gate dielectric, according to anexemplary embodiment;

FIG. 4 illustrates a cross-sectional view of the semiconductor structureand illustrates formation of a first gate conductor and a first blanketsacrificial layer, according to an exemplary embodiment;

FIG. 5 illustrates a cross-sectional view of the semiconductor structureand illustrates forming an organic polymer layer, according to anexemplary embodiment;

FIG. 6 illustrates a cross-sectional view of the semiconductor structureand illustrates selective removal of the first blanket sacrificial layerand the first gate conductor, according to an exemplary embodiment;

FIG. 7 illustrates a cross-sectional view of the semiconductor structureand illustrates selective crystallization of the gate dielectric,according to an exemplary embodiment;

FIG. 8 illustrates a cross-sectional view of the semiconductor structureand illustrates removal of remaining first blanket sacrificial layer andremaining first gate conductor, according to an exemplary embodiment;

FIG. 9 illustrates a cross-sectional view of the semiconductor structureand illustrates formation of a dipole layer, according to an exemplaryembodiment;

FIG. 10 illustrates a cross-sectional view of the semiconductorstructure and illustrates formation of a second gate conductor and asecond blanket sacrificial layer, according to an exemplary embodiment;

FIG. 11 illustrates a cross-sectional view of the semiconductorstructure and illustrates an annealing step, according to an exemplaryembodiment;

FIG. 12 illustrates a cross-sectional view of the semiconductorstructure and illustrates removal of the second blanket sacrificiallayer and the second gate conductor, according to an exemplaryembodiment;

FIG. 13 illustrates a cross-sectional view of the semiconductorstructure and illustrates formation of a work function metal, accordingto an exemplary embodiment; and

FIG. 14 illustrates a cross-sectional view of the FIG. 13, according toan exemplary embodiment.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some of the elements may be exaggeratedrelative to other elements for clarity. Further, where consideredappropriate, reference numbers may be repeated among the figures toindicate corresponding or analogous features.

DETAILED DESCRIPTION

Detailed embodiments of the claimed structures and methods are disclosedherein; however, it can be understood that the disclosed embodiments aremerely illustrative of the claimed structures and methods that may beembodied in various forms. This invention may, however, be embodied inmany different forms and should not be construed as limited to theexemplary embodiments set forth herein. In the description, details ofwell-known features and techniques may be omitted to avoid unnecessarilyobscuring the presented embodiments.

References in the specification to “one embodiment”, “an embodiment”,“an example embodiment”, etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

It will be understood that when an element as a layer, region orsubstrate is referred to as being “on” or “over” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

In the interest of not obscuring the presentation of embodiments of thepresent invention, in the following detailed description, someprocessing steps or operations that are known in the art may have beencombined together for presentation and for illustration purposes and insome instances may have not been described in detail. In otherinstances, some processing steps or operations that are known in the artmay not be described at all. It should be understood that the followingdescription is rather focused on the distinctive features or elements ofvarious embodiments of the present invention.

A nanosheet transistor may be formed from stacked nanosheets, withalternate layers of silicon and silicon germanium, which are then formedinto nanosheet stacks. A gate all around structure may be formed from ananosheet stack. As dimensions of the nanosheet transistor continue tobe reduced, close spacing between channels of a nanosheet transistormake it difficult to vary or adjust different threshold voltages.

The present invention generally relates to semiconductor manufacturingand more particularly to adjusting the threshold voltage of a nanosheettransistor. Adjusting the threshold voltage can be very challenging innanosheet manufacturing due to aggressive device scaling.

The inventors discovered the threshold voltage of a nanosheet transistorcan be adjusted by selectively crystallizing the gate dielectric layer.Additionally, the inventors discovered a dipole layer used to furtheradjust the threshold voltage does not diffuse into a crystallized gatedielectric. As such, the inventors discovered that selectivelycrystallizing the gate dielectric can be used to produce variousdifferent threshold voltages.

In particular, embodiments of the present invention disclose selectivelycrystalizing the gate dielectric layer during fabrication of a nanosheettransistor to modify the transistor's threshold voltage. Additionalembodiments of the present invention disclose selectively crystallizingthe gate dielectric to prevent a dipole layer from diffusing into thegate dielectric and further produce multiple different thresholdvoltages. Techniques involving selectively crystallizing the gatedielectric are described in detail below by referring to theaccompanying drawings in FIGS. 1-14, in accordance with an illustrativeembodiment.

Referring now to FIG. 1, a semiconductor structure 100 (hereinafter“structure”) at an intermediate stage of fabrication is shown accordingto an exemplary embodiment. FIG. 1 is a cross-sectional view of thestructure 100. The structure 100 of FIG. 1 may be formed or provided.Structure A and Structure B are the same at this point of fabricationand remain identical unless otherwise noted.

The structure 100 may include a nanosheet stack 20 separated by a bottomisolation layer 12 on a base substrate 10. The structure 100 may includea shallow trench isolation region (hereinafter “STI region”) 22. Itshould be noted that, while a limited number of nanosheet stack 20 aredepicted, any number of nanosheet stacks 20 may be formed.

The substrate 10 may be, for example, a bulk substrate, which may bemade from any of several known semiconductor materials such as, forexample, silicon, germanium, silicon-germanium alloy, and compound (e.g.III-V and II-VI) semiconductor materials. Non-limiting examples ofcompound semiconductor materials include gallium arsenide, indiumarsenide, and indium phosphide, or indium gallium arsenide. Typically,the substrate 10 may be approximately, but is not limited to, severalhundred microns thick. In other embodiments, the substrate 10 may be alayered semiconductor such as a silicon-on-insulator orSiGe-on-insulator, where a buried insulator layer, separates a basesubstrate from a top semiconductor layer.

A silicon germanium layer, not shown, may be formed on the substrate.The silicon germanium layer may, for example, have a germaniumconcentration about 60 atomic percent, although percentages greater than60% and less than 60% may be used. The silicon germanium layer can beformed using a deposition technique or an epitaxial growth technique.The silicon germanium layer will subsequently be removed selective tothe remaining layers of the nanosheet stack 20, as described below. Assuch, the silicon germanium layer can be made from other materials whichallow for their selective removal.

The terms “epitaxially growing and/or depositing” and “epitaxially grownand/or deposited” mean the growth of a semiconductor material on adeposition surface of a semiconductor material, in which thesemiconductor material being grown has the same crystallinecharacteristics as the semiconductor material of the deposition surface.In an epitaxial deposition technique, the chemical reactants provided bythe source gases are controlled and the system parameters are set sothat the depositing atoms arrive at the deposition surface of thesemiconductor substrate with sufficient energy to move around on thesurface and orient themselves to the crystal arrangement of the atoms ofthe deposition surface. Therefore, an epitaxial semiconductor materialhas the same crystalline characteristics as the deposition surface onwhich it is formed.

Examples of various epitaxial growth techniques include, for example,rapid thermal chemical vapor deposition (RTCVD), low-energy plasmadeposition (LEPD), ultra-high vacuum chemical vapor deposition (UHVCVD),atmospheric pressure chemical vapor deposition (APCVD) and molecularbeam epitaxy (MBE). The temperature for epitaxial deposition typicallyranges from approximately 550° C. to approximately 900° C. Althoughhigher temperature typically results in faster deposition, the fasterdeposition may result in crystal defects and film cracking. Theepitaxial growth the first and second semiconductor materials thatprovide the sacrificial semiconductor material layers and thesemiconductor channel material layers, respectively, can be performedutilizing any well-known precursor gas or gas mixture. Carrier gaseslike hydrogen, nitrogen, helium and argon can be used.

The nanosheet stack 20 includes vertically aligned alternating layers ofsacrificial semiconductor material layer 16 and semiconductor channelmaterial layer 18. The nanosheet stack 20 is formed on the silicongermanium layer. In FIG. 1, and only by way of an example, the nanosheetstack 20 includes three layers of sacrificial semiconductor materiallayer 16 and three layers of semiconductor channel material layer 18.The material stacks that can be employed in embodiments of the presentinvention are not limited to the specific embodiment illustrated inFIG. 1. The nanosheet stack 20 can include any number of sacrificialsemiconductor material layers 16 and semiconductor channel materiallayers 18. The nanosheet stack 20 is used to produce a gate-all-arounddevice that includes vertically stacked semiconductor channel materialnanosheets for a p-channel field-effect transistor (PFET) or ann-channel field-effect transistor (NFET) device.

Each sacrificial semiconductor material layer 16 is composed of a firstsemiconductor material which differs in composition from at least anupper portion of the substrate 10 and differs in composition from thesilicon germanium layer with 60% germanium. In an embodiment, eachsacrificial semiconductor material layer 16 may have a germaniumconcentration less than 50 atomic percent. In another example, eachsacrificial semiconductor material layer 16 may have a germaniumconcentration ranging from about 20 atomic percent to about 40 atomicpercent. Each sacrificial semiconductor material layer 16 can be formedusing known deposition techniques or an epitaxial growth technique asdescribed above.

Each semiconductor channel material layer 18 is composed of a secondsemiconductor material which differs in composition from at least anupper portion of the substrate 10, differs in composition from thesilicon germanium layer and differs in composition from the sacrificialmaterial layer 16. Each semiconductor channel material layer 18 has adifferent etch rate than the first semiconductor material of sacrificialsemiconductor material layers 16 and has a different etch rate than thesilicon germanium layer. The second semiconductor material can be, forexample, silicon. The second semiconductor material, for eachsemiconductor channel material layer 18, can be formed using knowndeposition techniques or an epitaxial growth technique as describedabove.

The nanosheet stack 20 (16, 18) can be formed by sequential epitaxialgrowth of alternating layers of the first semiconductor material and thesecond semiconductor material.

The sacrificial semiconductor material layers 16 of the nanosheet stack20 may have a thickness ranging from about 5 nm to about 12 nm, whilethe semiconductor channel material layers 18 of the semiconductor stack20 may have a thickness ranging from about 3 nm to about 12 nm. Eachsacrificial semiconductor material layer 16 may have a thickness that isthe same as, or different from, a thickness of each semiconductorchannel material layer 18. In an embodiment, each sacrificialsemiconductor material layer 16 has an identical thickness. In anembodiment, each semiconductor channel material layer 18 has anidentical thickness.

The nanosheet stack 20 is formed by patterning the sacrificialsemiconductor material layers 16 and the semiconductor channel materiallayers 18. The silicon germanium layer may be patterned simultaneouslywith the nanosheet stack 20. More specifically, portions of thesacrificial semiconductor material layers 16, the semiconductor channelmaterial layers 18, and the silicon germanium layer are etched using ananisotropic etching technique, such as, for example, reactive ionetching (ME), and stopping on the substrate 10 and the STI regions 22.The nanosheet stack 20 includes alternating nanosheets of remainingportions of each sacrificial semiconductor material layer 16 and eachsemiconductor channel material layer 18 all on top of a remainingportion of the silicon germanium layer. After etching, sidewalls of eachsacrificial semiconductor material layer 16 are vertically aligned tosidewalls of each semiconductor channel material layer 18, and tosidewalls of the silicon germanium layer.

Adjacent nanosheet stacks 20 may be isolated from one another by regionsof dielectric material such as, for example, the STI regions 22. The STIregions 22 may be formed using known patterning and depositiontechniques.

As previously mentioned, the silicon germanium layer is then selectivelyremoved using one or more known techniques. In doing so, the silicongermanium layer is removed selective to the semiconductor channelmaterial layers 18, the semiconductor channel material layers 18 and theSTI regions 22. For example, a wet etching technique can be used toselectively remove the silicon germanium layer. The wet etchingtechnique may employ special chemical solutions including, for example,tetramethylammonium hydroxide (TMAH) solution, potassium hydroxide (KOH)solution, and ethylene diamine and pyrocatechol (EDP) solution.Alternatively, for example, a wet etching technique that relies on amixture solution of HF-HNO3-H2SO4 may be used.

The bottom isolation layer 12 may be formed on the substrate 10 andbelow the nanosheet stacks 20 in a gap created by removal of the silicongermanium layer. The bottom isolation layer 12 may be formed byconformally depositing a dielectric material, followed by one or moreetch or recessing steps.

The bottom isolation layer 12 may be composed of silicon dioxide,undoped silicate glass (USG), fluorosilicate glass (FSG),borophosphosilicate glass (BPSG), a spin-on low-k dielectric layer, achemical vapor deposition (CVD) low-k dielectric layer or anycombination thereof. The term “low-k” as used throughout the presentapplication denotes a dielectric material that has a dielectric constantof less than silicon dioxide. In another embodiment, a self-planarizingmaterial such as a spin-on glass (SOG) or a spin-on low-k dielectricmaterial such as SiLK™ can be used as the bottom isolation layer 12.Using a self-planarizing dielectric material as the bottom isolationlayer 12 may avoid the need to perform subsequent etching or recessing.SiLK™ is a trademark of Dow Chemical Company.

In some embodiments, as shown, the bottom isolation layer 12 may beselectively etched such that vertical sides of the bottom isolationlayer 12 align with the nanosheet stack 20, and a top surface of the STIregions 22 is exposed. An anisotropic etching technique, such as, forexample, reactive ion etching (RIE) may be used to etch the bottomisolation 12. After etching, sidewalls of each sacrificial semiconductormaterial layer 16 are vertically aligned to sidewalls of eachsemiconductor channel material layer 18, and to sidewalls of the bottomisolation layer 12. In other embodiments, not shown, the bottomisolation layer 12 may remain a continuous layer extending from onenanosheet stack 20 to the next and covering the STI regions 22.

Referring now to FIG. 2, the structure 100 is shown according to anexemplary embodiment. As shown in FIG. 2, the sacrificial semiconductormaterial layers 16 are selectively removed using one or more etchingtechniques. In doing so, the sacrificial semiconductor material layers16 are removed selective to the semiconductor channel material layers18, the bottom isolation layer 12 and the STI regions 22. As illustratedin FIG. 2, the remaining semiconductor channel material layers 18 areshown suspended and are supported on both ends by additional portions ofthe structure 100 which are not shown.

For example, a wet etching technique can be used to selectively removethe sacrificial semiconductor material layers 16. The wet etchingtechnique may employ special chemical solutions including, for example,tetramethylammonium hydroxide (TMAH) solution, potassium hydroxide (KOH)solution, and ethylene diamine and pyrocatechol (EDP) solution.

Alternatively, for example, a wet etching technique that relies on amixture solution of HF-HNO3-H2SO4 may be used.

Referring now to FIG. 3, the structure 100 is shown according to anexemplary embodiment. As shown in FIG. 3, a gate dielectric 24 is formedin each cavity and surrounding suspended portions of the semiconductorchannel material layers 18. The gate dielectric 24 further covers theSTI regions 22 and the bottom isolation layer 12 as illustrated. Inpractice, the gate oxide 24 is deposited directly on an interfaciallayer (not shown). The interfacial layer will be a native oxide such as,for example, silicon oxide.

The gate dielectric 24 can be an oxide, nitride, and/or oxynitride. Inan example, the gate dielectric 24 can be a high-k material. Exemplaryhigh-k dielectrics include, but are not limited to, HfO₂, ZrO₂, La₂O₃,Al₂O₃, TiO₂, SrTiO₃, LaAlO₃, Y₂O₃, HfO_(x)N_(y), ZrO_(x)N_(y),La₂O_(x)N_(y), Al₂O_(x)N_(y), TiO_(x)N_(y), SrTiO_(x)N_(y),LaAlO_(x)N_(y), Y₂O_(x)N_(y), SiON, SiN_(x), a silicate thereof, and analloy thereof. Each value of x is independently from 0.5 to 3 and eachvalue of y is independently from 0 to 2. In some embodiments, amultilayered gate dielectric structure including different gatedielectric materials, e.g., silicon dioxide, and a high-k gatedielectric, can be formed and used as the gate dielectric 24.

The gate dielectric 24 can be formed by any deposition techniqueincluding, for example, atomic layer deposition (ALD), chemical vapordeposition (CVD), plasma enhanced chemical vapor deposition (PECVD),physical vapor deposition (PVD), sputtering, or other like depositiontechniques. In an embodiment, the gate dielectric 24 can have athickness ranging from approximately 1 nm to approximately 10 nm. Otherthicknesses that are lesser than, or greater than, the aforementionedthickness range can also be employed for the gate dielectric 24.

Referring now to FIG. 4, the structure 100 is shown according to anexemplary embodiment. As shown in FIG. 4, a first sacrificial gate 26 isformed on top of and covering the gate dielectric 24. For example, thefirst sacrificial gate 26 is formed in each cavity and surroundssuspended portions of the semiconductor channel material layers 18. Alsoas shown in FIG. 4, a first blanket sacrificial layer 28 may be formed,covering the first sacrificial gate 26, and may fill an area betweenadjacent nanosheet stacks 20.

The first sacrificial gate 26 can include any material, for example,polysilicon, amorphous silicon, an elemental metal (e.g., tungsten,titanium, tantalum, aluminum, nickel, ruthenium, palladium andplatinum), an alloy of at least two elemental metals or multilayeredcombinations thereof.

The first sacrificial gate 26 can be formed by any deposition techniqueincluding, for example, chemical vapor deposition (CVD), plasma enhancedchemical vapor deposition (PECVD), physical vapor deposition (PVD),sputtering, atomic layer deposition (ALD) or other like depositiontechniques. When a metal silicide is formed, a conventional silicidationtechnique is used. In an embodiment, the first sacrificial gate 26 canhave a thickness ranging from approximately 1 nm to approximately 50 nm,and more preferably ranging from approximately 3 nm to approximately 10nm. Other thicknesses that are lesser than, or greater than, theaforementioned thickness range can also be employed for the firstsacrificial gate 26. In an embodiment, the thickness of the firstsacrificial gate 26 is deposited with a thickness sufficient to fill, orsubstantially fill, the spaces between adjacent semiconductor channelmaterial layers 18, and completely surround each of the semiconductorchannel material layers 18.

The first blanket sacrificial layer 28 can include any oxygen blockingmaterial including, for example, amorphous silicon, polycrystallinesilicon, amorphous carbon, amorphous germanium, polycrystallinegermanium, or polycrystalline silicon-germanium made. The first blanketsacrificial layer 28 can be formed utilizing a deposition processincluding, for example, chemical vapor deposition (CVD), plasma enhancedchemical vapor deposition (PECVD), physical vapor deposition (PVD),sputtering, atomic layer deposition (ALD) or other like depositiontechniques.

Referring now to FIG. 5, the structure 100 is shown according to anexemplary embodiment. As shown in FIG. 5, an organic polymer layer 30may be selectively formed. The organic polymer layer 30 may coverselected nanosheet stacks 20, and may not cover other nanosheet stacks20. As shown in FIG. 5, the organic polymer layer 30 is used to coverthe two nanosheet stacks 20 in Structure B, and not the two nanosheetstacks in Structure A.

The organic polymer layer 30 may be formed by a blanket deposition usingtypical deposition techniques, for example, atomic layer deposition(ALD), chemical vapor deposition (CVD), plasma enhanced chemical vapordeposition (PECVD), physical vapor deposition (PVD), sputtering, orother like deposition techniques. The material of the organic polymerlayer 30 may include a photo-sensitive organic polymer including alight-sensitive material. The organic polymer may include epoxy resin,phenol resin, polyacrylate resin, polyamide resin, polyimide resin,unsaturated polyester resin, polyphenylenesulfide resin,polyphenylenether resin, or benzocyclobutene (BCB). The material of theorganic polymer layer 30 may be selected to be compatible with the firstsacrificial gate 26 and the first blanket sacrificial layer 28.Specifically, the materials are chosen such that one or more of thefirst sacrificial gate 26, the first blanket sacrificial layer 28, orthe organic polymer layer 30 may be etched or recessed selective to oneanother.

At this point of fabrication, the Structures A and B are different.Specifically, the organic polymer layer 30 covers only the Structure B,and does not cover the Structure A.

Referring now to FIG. 6, the structure 100 is shown according to anexemplary embodiment. As shown in FIG. 6, the first blanket sacrificiallayer 28 and the first sacrificial gate 26 may be selectively removedfrom the Structure A. Subsequently, the organic polymer layer 30 may beremoved from the Structure B.

As described above, a wet etching technique can be used to selectivelyremove the first blanket sacrificial layer 28 and the first gate 26selective to the organic polymer layer 30. The wet etching technique mayemploy special chemical solutions including, for example,tetramethylammonium hydroxide (TMAH) solution, potassium hydroxide (KOH)solution, and ethylene diamine and pyrocatechol (EDP) solution.Alternatively, for example, a wet etching technique that relies on amixture solution of HF-HNO3-H2SO4 may be used. The etching may beperformed in one or more steps.

Following the removal of the first blanket sacrificial layer 28 and thefirst sacrificial gate 26 in the Structure A, the organic polymer layer30 may be removed from the Structure B. A wet etching technique may beused to selectively remove the organic polymer layer 30, selective tothe first blanket sacrificial layer 28 of the Structure B, the firstsacrificial gate 26 and the gate dielectric 24.

At this point of fabrication, the Structures A and B are different.Specifically, the first blanket sacrificial layer 28 and the firstsacrificial gate 26 remain in the Structure B, while neither the firstblanket sacrificial layer 28 nor the first sacrificial gate 26 remain onthe Structure A.

Referring now to FIG. 7, the structure 100 is shown according to anexemplary embodiment. As shown in FIG. 7, the structure 100 may beexposed to a spike anneal, a high temperature anneal, a laser spikeanneal, or another type of an anneal, to crystalize exposed portions ofgate dielectric 24. In the structure A, the gate dielectric 24 becomes acrystallized gate dielectric 32 with different properties than the gatedielectric 24. Because the gate dielectric 24 in the Structure B isprotected by the first sacrificial gate 26 and the first blanketsacrificial layer 28, and thus not exposed, it will not be crystallizedby the spike anneal.

In an embodiment, the annealing technique may include subjecting thestructure 100 to an elevated temperature, ranging from approximately800° C. to approximately 1250° C., for approximately 1 ms toapproximately 500 ms. In another embodiment, a high-temperature rapidthermal anneal (RTA) technique may be used. Typically, high temperaturescannot be used during fabrication due to risk of damaging a gate metalor work function metal; however, in the present case neither the gatemetal nor the work function metal have been formed yet.

The threshold voltage of a nanosheet transistor is dependent upon thematerial and structure of the nanosheet stack 20, and also depends upona thickness and composition of the gate dielectric surrounding each ofthe semiconductor channel material layers 18 of the nanosheet stack 20.Prior to the laser spike anneal, the Structure A and the Structure Beach had the same gate dielectric layer (i.e. the gate dielectric 24)formed at the same time, with the same original thickness andcomposition, and thus would have the same or substantially the samethreshold voltage. However, after the laser spike anneal step, theStructure A has the crystallized gate dielectric 32, which has adifferent composition, compared to the gate dielectric 24 of theStructure B. As such, the threshold voltage for the Structure A will bedifferent from the threshold voltage for the Structure B.

The use of spike laser anneal allows for selective crystallizationwithout additional processing steps. Traditionally, additionalprocessing steps would be required to form the Structure A and theStructure B with different gate dielectrics and different thresholdvoltages.

Referring now to FIG. 8, the structure 100 is shown according to anexemplary embodiment. As shown in FIG. 8, portions of the first blanketsacrificial layer 28 and the first sacrificial gate 26 remaining on theStructure B are removed.

As described above, a wet etching technique can be used to selectivelyremove remaining portions of the first blanket sacrificial layer 28 andthe first sacrificial gate 26 selective to the gate dielectric 24 andthe crystallized gate dielectric 32.

Referring now to FIG. 9, the structure 100 is shown according to anexemplary embodiment. As shown in FIG. 9, a dipole layer 34 may beformed.

In the Structure A, the dipole layer 34 is formed on top of thecrystallized gate dielectric 32 surrounding the semiconductor channelmaterial layers 18 of the nanosheet stack 20. The dipole layer 34 isfurther formed on top of the crystallized gate dielectric 32 coveringthe bottom isolation layer 12 and the STI regions 22.

In the Structure B, the dipole layer 34 is formed on top of the gatedielectric 24 surrounding the semiconductor channel material layers 18of the nanosheet stack 20. The dipole layer 34 is further formed on topof the gate dielectric 24 covering the bottom isolation layer 12 and theSTI regions 22.

The dipole layer 34 may by formed by a blanket deposition using typicaldeposition techniques, for example, atomic layer deposition (ALD),chemical vapor deposition (CVD), plasma enhanced chemical vapordeposition (PECVD), physical vapor deposition (PVD), sputtering, orother like deposition techniques. The material of the dipole layer 34may include any suitable dipole layer know to a person having ordinaryskill in the art. For example, in some embodiments the dipole layer 34may include lanthanum oxide (La₂O₃) or aluminum oxide (Al₂O₃). In someembodiments, lanthanum oxide (La₂O₃) is commonly used for NFET devicesand aluminum oxide (Al₂O₃) is commonly used for PFET devices. In otherembodiments, some combination of lanthanum oxide (La₂O₃) and aluminumoxide (Al₂O₃) can be used for either NFET devices or PFET devices inorder to achieve desired device characteristics.

Adjusting the material of the dipole layer 34 may be another method toalter or change the threshold voltage for the nanosheet transistors. Inan embodiment, for example, a lanthanum oxide (La₂O₃) dipole layer (34)may typically lower the threshold voltage of an NFET device and maytypically raise the threshold voltage of a PFET device. In an alternateembodiment, for example, a aluminum oxide (Al₂O₃) dipole layer (34) maytypically increase the threshold voltage of an NFET device and maytypically lower the threshold voltage of a PFET device.

In some embodiments, it may be desirable to selectively deposit thedipole layer 34 over select nanosheet stacks 20, and not others.

Referring now to FIG. 10, the structure 100 is shown according to anexemplary embodiment. As shown in FIG. 10, a second sacrificial gate 36is formed.

In the Structure A, the second sacrificial gate 36 is formed in eachcavity and surrounding suspended portions of the semiconductor channelmaterial layers 18, the crystallized gate dielectric 32 and the dipolelayer 34. Meanwhile, in the Structure B the second sacrificial gate 36is formed in each cavity and surrounding suspended portions of thesemiconductor channel material layers 18, the gate dielectric 24 and thedipole layer 34. In some cases, the second sacrificial gate 36 may alsocover the STI regions 22 and surfaces of the bottom isolation layer 12.

Also as shown in FIG. 10, a second blanket sacrificial layer 38 may beformed, covering the second sacrificial gate conductor 36, and may fillan area between adjacent nanosheet stacks 20.

The second sacrificial gate conductor 36 may be formed and includematerials as described above for the first sacrificial gate 26. Thesecond blanket sacrificial layer 38 may be formed and include materialsas described above for the first blanket sacrificial layer 28.

Referring now to FIG. 11, the structure 100 is shown according to anexemplary embodiment. As shown in FIG. 11, an annealing step isperformed to cause the dipole layer 34 to diffuse into the gatedielectric 24 in Structure B only. In contrast, the dipole layer 34 doesnot diffuse into the crystallized gate dielectric 32 of the Structure A,but instead remains on top of the gate dielectric layer 24.

More specifically, the Structure A remains substantially the same beforeand after the anneal, because the crystalized gate dielectric 32prevents diffusion of the dipole layer 34. As such, in the Structure Aonly, the crystallized gate dielectric 32 separates the dipole layer 34from the semiconductor channel material layers 18. In contrast,annealing causes the dipole layer 34, to diffuse into the (amorphous)gate dielectric layer 24 of the Structure B. After annealing, the dipolematerial, for example lanthanum oxide (La₂O₃), aluminum oxide (Al₂O₃),or both, from the dipole layer 34 can be detected within the gate layer24 and at the interface between the gate later 24 and the interfaciallayer. Annealing may include a spike anneal, a high temperature anneal,or a laser spike anneal, or another type of an anneal, as describedherein above.

Causing the dipole layer 34 to diffuse into the gate dielectric layer 24of the Structure B, is another example of how selective crystallization,or lack thereof in the present example, can be used to adjust or changethe threshold voltage of a nanosheet device in accordance with thedisclosed embodiments. For example, the Structure B with the diffuseddipole material can be designed and fabricated with a differentthreshold than the Structure A. Even more unique, the discloseembodiments teach how to fabricate nanosheet devices with differentthreshold voltages using selective crystallization of the gatedielectric layer 24.

Referring now to FIG. 12, the structure 100 is shown according to anexemplary embodiment. As shown in FIG. 12, the second blanketsacrificial layer 38 and the second sacrificial gate 36 may be removedfrom the Structure 100. Additionally, the dipole layer 34 may be removedfrom the Structure A.

As described above, one or more etching techniques, for example a wetetching technique, can be used to selectively remove the second blanketsacrificial layer 38, the second gate conductor 36 from both theStructure A and the Structure B. In the Structure A the wet etch willremove the dipole layer 34 selective to the crystallized gate dielectric32. In the Structure B, the wet etch may remove a portion of the gatedielectric 24; however, the diffused dipole material remains withinremaining portions of the gate dielectric 24 and along the interfacebetween the gate dielectric 24 and the interfacial layer.

Referring now to FIGS. 13 and 14, the structure 100 is shown accordingto an exemplary embodiment. The Structure A of FIG. 14 is across-section view perpendicular to the cross-sectional view of theStructure A illustrated in FIG. 13, along section line A-A. TheStructure B of FIG. 14 is a cross-section view perpendicular to thecross-sectional view of the Structure B illustrated in FIG. 13, alongsection line B-B. As shown in FIGS. 13 and 14, a work function metal 40may be formed.

The work function metal 40 may be formed as part of a traditional gateprocess for the nanosheet FET formed from the gate stack 20. Due to thedifferences in the Structure A and the Structure B, the same workfunction metal 40 may be used for both. The differences in the twostructures, including the crystallized gate dielectric 32 of theStructure A compared to the gate dielectric 24 of the Structure A, andthe diffused dipole material of the Structure B, may be used to producetwo or more transistors having different threshold voltages, even withthe same work function metal 40.

In an embodiment, either of the Structures A, B, may be used as eitheran NFET device, or as a PFET device.

The work function metal 40 may be conformally formed on the structure100, according to an exemplary embodiment. The work function metal 40may be deposited using typical deposition techniques, for example,atomic layer deposition (ALD), molecular layer deposition (MLD), andchemical vapor deposition (CVD).

The material chosen for the work function metal 40 may be selected basedon whether an NFET device or a PFET device is desired. In an embodiment,the work function metal 40 of a PFET device may include a metal nitride,for example, titanium nitride or tantalum nitride, titanium carbidetitanium aluminum carbide, or other suitable materials known in the art.In an embodiment, the work function metal 40 of an NFET device mayinclude, for example, titanium aluminum carbide or other suitablematerials known in the art. In an embodiment, the work function metal 40may include one or more layers to achieve desired devicecharacteristics.

As shown in FIG. 14, an inner spacer 42 may be formed, a source drainregions 44, and a gate spacer 46 may all be formed according to knowtechniques. Formation of the source drain regions 44 and the gate spacer46 is typically completed prior to beginning fabrication illustrated inFIG. 1.

Within the structure 100, there may be different combinations of gatedielectric layers and dipole layers to produce devices with differentthreshold voltages.

In accordance with the embodiments described herein, selectivecrystallization of the gate dielectric yields at least four differenttechniques to control or adjust the threshold voltages of nanosheetdevices are disclosed. For example, a first nanosheet transistor havinga first threshold voltage would include only the gate dielectric 24alone without crystallization and without the dipole layer; a secondnanosheet transistor having a second threshold voltage would includeonly the crystallized gate dielectric 32 alone without the dipole layer34; a third nanosheet transistor having a third threshold voltage wouldinclude the dipole layer 34 diffused into the gate dielectric 24, asillustrated in Structure B; and a fourth nanosheet transistor having afourth threshold voltage would include only the dipole layer 34 on topof the crystalized gate dielectric 32, as illustrated in Structure A.

As briefly mentioned above, embodiments of the present inventiondisclose more efficient and less invasive techniques to fabricatenanosheet transistors with different threshold voltages. For example,conventional manufacturing techniques require depositing differentmaterials, for example, different gate dielectrics, to achieve differentthreshold voltages. Doing so inherently requires multiple deposition,masking, and etching techniques which can be invasive and harmful tosurrounding structures. Instead, embodiments of the present inventionprovide techniques to achieve different threshold voltages whileminimizing extra process steps, thereby minimizing collateral damage tosurrounding structures. For example, embodiments of the presentinvention begin with a single gate dielectric material (24) and deviceswith different threshold voltages can be achieved by modifying the gatedielectric through crystallization.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the invention.The terminology used herein was chosen to best explain the principles ofthe embodiment, the practical application or technical improvement overtechnologies found in the marketplace, or to enable others of ordinaryskill in the art to understand the embodiments disclosed herein.

What is claimed is:
 1. A semiconductor structure comprising: nanosheetstacks on a substrate, each nanosheet stack comprising alternatinglayers of a sacrificial semiconductor material and a semiconductorchannel material vertically aligned and stacked one on top of another;and a crystallized gate dielectric layer surrounding the semiconductorchannel layers of a first subset of the nanosheet stacks; a dipole layeron top of the crystallized gate dielectric and surrounding the layers ofsemiconductor channel material of the first subset of the nanosheetstacks; and a gate dielectric modified by a diffused dipole materialsurrounding the semiconductor channel layers of a second subset of thenanosheet stacks.
 2. The semiconductor structure according to claim 1,further comprising: a first gate conductor layer orthogonal to thenanosheet stack, the first gate conductor wraps around the semiconductorchannel material layers of the nanosheet stack.
 3. The semiconductorstructure according to claim 1, wherein crystallizing the gatedielectric comprises annealing the gate dielectric to form thecrystallized gate dielectric.
 4. The semiconductor structure accordingto claim 1, wherein the sacrificial semiconductor material layerscomprise silicon germanium.
 5. The semiconductor structure according toclaim 1, further comprising: source drain regions extending laterallyfrom either end of the semiconductor channel material layers of thenanosheet stack.
 6. The semiconductor structure according to claim 1,wherein the crystallized gate dielectric layer directly contacts andcovers shallow trench isolation regions between adjacent nanosheetstacks.
 7. A semiconductor structure comprising: nanosheet stacks on asubstrate, each nanosheet stack comprising alternating layers of asacrificial semiconductor material and a semiconductor channel materialvertically aligned and stacked one on top of another; and a crystallizedgate dielectric surrounding the semiconductor channel layers of thenanosheet stacks.
 8. The semiconductor structure according to claim 7,further comprising: a dipole layer on top of the crystallized gatedielectric and surrounding the layers of semiconductor channel materialof a subset of the nanosheet stacks.
 9. The semiconductor structureaccording to claim 7, further comprising: a first gate conductor layerorthogonal to the nanosheet stack, the first gate conductor wraps aroundthe semiconductor channel material layers of the nanosheet stack. 10.The semiconductor structure according to claim 7, wherein crystallizingthe gate dielectric comprises annealing the gate dielectric to form thecrystallized gate dielectric.
 11. The semiconductor structure accordingto claim 7, wherein the sacrificial semiconductor material layerscomprise silicon germanium.
 12. The semiconductor structure according toclaim 7, further comprising: source drain regions extending laterallyfrom either end of the semiconductor channel material layers of thenanosheet stack.
 13. The semiconductor structure according to claim 7,wherein the crystallized gate dielectric layer directly contacts andcovers shallow trench isolation regions between adjacent nanosheetstacks.
 14. A method comprising: forming nanosheet stacks on asubstrate, each nanosheet stack comprising alternating layers of asacrificial semiconductor material and a semiconductor channel materialvertically aligned and stacked one on top of another; removing thesacrificial semiconductor material layers of the set of nanosheetstacks; forming a gate dielectric surrounding the semiconductor channellayers of the nanosheet stacks; and crystalizing the gate dielectric ofa subset of the nanosheet stacks.
 15. The method according to claim 14,further comprising: forming a dipole layer surrounding the gatedielectric which surrounds the semiconductor channel layers of a subsetof the nanosheet stacks.
 16. The method according to claim 14, furthercomprising: diffusing the dipole layer into the gate dielectricsurrounding the semiconductor channel layers of the subset of thenanosheet stacks.
 17. The method according to claim 14, furthercomprising: forming a first gate conductor layer orthogonal to thenanosheet stack, the first gate conductor wraps around the semiconductorchannel material layers of the nanosheet stack.
 18. The method accordingto claim 14, wherein crystallizing the gate dielectric comprisesannealing the gate dielectric to form the crystallized gate dielectric.19. The method according to claim 14, wherein the sacrificialsemiconductor material layers comprise silicon germanium.
 20. The methodaccording to claim 14, further comprising: forming source drain regionsextending laterally from either end of the semiconductor channelmaterial layers of the nanosheet stack.